docs: def buffer(self, item):
          """
          Receive an item and write it.
          """
          key = self.get_key_from_item(item)
          if not self.grouping_info.is_first_file_item(key):
              self.items_group_files.add_item_separator_to_file(key)
          self.grouping_info.ensure_group_info(key)
          self.items_group_files.add_item_to_file(item, key)